The invention relates to radio systems that apply frequency hopping spread spectrum techniques, and more particularly to methods and apparatuses for speeding up the frequency-hop synchronization between units of frequency hopping systems at the time of connection setup.
In the last several decades, progress in radio and Very Large Scale Integrated circuit (VLSI) technology has fostered widespread use of radio communications in consumer applications. Portable devices, such as mobile radiotelephones, can now be produced having acceptable cost, size and power consumption.
Although wireless technology is today focused mainly on voice communications (e.g., with respect to handheld radios), this field will likely expand in the near future to provide greater information flow to and from other types of nomadic devices and fixed devices. More specifically, it is likely that further advances in technology will provide very inexpensive radio equipment which can be easily integrated into many devices. This will reduce the number of cables currently used for many applications. For example, radio communication can eliminate or reduce the number of cables used to connect master devices with their respective peripherals.
The aforementioned radio communications will require an unlicensed band with sufficient capacity to allow for high data rate transmissions. A suitable band is the so-called Industrial, Scientific and Medical (ISM) band at 2.4 GHz, which is globally available. The ISM band provides 83.5 MHZ of radio spectrum.
To allow different radio networks to share the same radio medium without coordination, signal spreading is usually applied. In fact, the Federal Communications Commission (FCC) in the United States currently requires radio equipment operating in the 2.4 GHz band to apply some form of spectrum spreading technique when the transmit power exceeds about 0 dBm. Spread spectrum communication techniques, which have been around since the days of World War II, are of interest in today""s commercial applications because they provide robustness against interference, which allows for multiple signals to occupy the same bandwidth at the same time.
Spreading can either be at the symbol level by applying direct-sequence (DS) spread spectrum techniques or at the channel level by applying frequency hopping (FH) spread spectrum techniques. In DS spread spectrum, the informational data stream to be transmitted is impressed upon a much higher rate data stream known as a signature sequence. Typically, the signature sequence data are binary, thereby providing a bit stream. One way to generate this signature sequence is with a pseudo-noise (PN) process that appears random, but can be replicated by an authorized receiver. The informational data stream and the high bit rate signature sequence stream are combined to generate a stream of so-called xe2x80x9cchipsxe2x80x9d by multiplying the two bit streams together, assuming the binary values of the two bit streams are represented by +1 or xe2x88x921. This combination of the higher bit rate signal with the lower bit rate data stream is called spreading the informational data stream signal. Each informational data stream or channel is allocated a unique signature sequence. At the receiver, the same unique signature sequence is used to recover the underlying informational data stream signal.
In frequency hopping systems, the spreading is achieved by transmitting the informational data stream over ever-changing radio frequencies. For each communication, the particular frequencies used by both the transmitter and receiver are determined by a predefined frequency hop sequence.
The use of frequency hopping is attractive for the radio applications mentioned above because it more readily allows the use of cost effective radios. However, FH systems are less appropriate when a connection has to be established quickly. This is because, in order to communicate, a FH transmitter and receiver must be hop synchronized so that both use the same hop channel at the same time. Prior to synchronization, the transmitter may have no knowledge of which hop channel the recipient will listen in on and when. During the synchronization process, the uncertainty in both time and frequency must be resolved. This problem becomes more difficult when the number of hop frequencies used in the FH system increases because when the number of frequencies in the synchronization procedure increases, the uncertainty in frequency increases and the amount of time required to establish synchronization will increase as well, thus delaying the access time.
There is therefore a need for techniques to reduce the access delay in FH systems such that the uncertainty in time and frequency can be resolved quickly.
The foregoing and other objects are achieved in methods and apparatuses for use in communication systems. In accordance with one aspect of the invention, a hybrid frequency hopping/direct sequence spread-spectrum mode of communication is employed during startup of a connection. Furthermore, a pure frequency hopping spread-spectrum mode of communication is employed during a connected mode of the connection.
In another aspect of the invention, the chip rate utilized in the hybrid frequency hopping/direct sequence spread-spectrum transmission mode of communication is identical to a bit transmission rate used by the pure frequency hopping spread-spectrum transmission mode of communication.
In still another aspect of the invention, a page message is sent by means of the hybrid frequency hopping/direct sequence spread-spectrum transmission mode of communication during startup of the connection. In one embodiment, the page message may comprise a fixed chip sequence that is unique to a radio unit being paged. In an alternative embodiment, the page message may comprise a fixed chip sequence that is common to all radio units in the radio system.
In yet another aspect of the invention, a chip rate of the page message may be identical to a bit rate used during the pure frequency hopping spread-spectrum transmission mode of communication.
In still another aspect of the invention, a sliding correlator may be used to despread the page message that is transmitted during startup of the connection. In some embodiments, the sliding correlator may be implemented as a tapped delay line.
In yet another aspect of the invention, a processing gain of the hybrid frequency hopping/direct sequence spread-spectrum transmission mode of communication may equal or exceed 17 dB.
In still another aspect of the invention, the hybrid frequency hopping/direct sequence spread-spectrum transmission mode of communication used during startup of the connection utilizes a first hop rate, and the pure frequency hopping spread-spectrum transmission mode of communication during a connected mode of the connection utilizes a second hop rate, wherein the first hop rate is a multiple of the second hop rate.
In yet another aspect of the invention, the pure frequency hopping spread-spectrum transmission mode of communication utilized during a connected mode of the connection uses a hop sequence that comprises at least 75 hop frequencies.
In still another aspect of the invention, the hybrid frequency hopping/direct sequence spread-spectrum transmission mode of communication used during startup of the connection utilizes a first hop sequence; and the pure frequency hopping spread-spectrum transmission mode of communication used during a connected mode of the connection uses a second hop sequence, wherein the first hop sequence consists of fewer hop frequencies than are defined by the second hop sequence.
In yet another aspect of the invention, a transceiver may be operated in a radio system by employing a hybrid frequency hopping/direct sequence spreadspectrum mode of communication during a first mode of operation on a connection; and employing a pure frequency hopping spread-spectrum mode of communication during a second mode of operation of the connection, wherein a first hop sequence for use by the hybrid frequency hopping/direct sequence spread-spectrum mode of communication consists of fewer hop frequencies than are defined by a second hop sequence for use by the pure frequency hopping spread-spectrum mode of communication.